MU HER (Mu Herculis). Southeast of the Keystone of Hercules, find a prominent third magnitude (3.42) star
known best just as Mu Herculis. Most stars shine well in the
nightly sky because they are truly luminous. This one, however, is
notable because it is so nearby, only 27.4 light years away (so
close in fact that we can actually express it in tenths of a light
year). The star -- a class G (G5) subgiant -- in fact is not that
much different from our own Sun, and closely
shows us what the Sun will look like after it has ceased hydrogen
fusion in its core and has begun to die. The big difference is
that Mu is not so much a star as a multiple-star "system." First,
though, back to Mu itself. Slightly cooler than the Sun, the
temperature measures range from 5414 to 5603 Kelvin, for an average
of 5506 (compared with the Sun's 5780 K). After a slight
correction for infrared radiation, the luminosity comes in at 2.7
solar, which combined with temperature gives a radius between 1.77
and 1.86 solar. Remarkably, Mu is close enough for us to be able
to measure the angular diameter of its disk, which yields 1.76
solar, right on the mark. From these parameters and the theory of
stellar evolution, the mass is but 1.1 times that of the Sun. The
relatively large radius is mostly the result of old age. As stars
age, they at first swell, and Mu is preparing to become a true
"giant." In spite of its age (stellar rotations slow as stars get
old), Mu is spinning at least 10 times the solar rotation velocity,
giving it a rotation period less than 4.3 days. As a result, Mu is
still magnetically active and radiates X-rays. Half a minute of
arc away (at least 300 Astronomical Units) is a pair of orbiting
cool M (M3 and M4, called "B" and "C," 10th and 11 magnitude)
dwarfs averaging but 2.2 AU apart (about as far as a typical
asteroid is from the Sun) that spin around each other every 43
years, which gives a combined mass of 0.8 solar, a bit high given
the M3-M4 spectral classes. A modest orbital eccentricity actually
swings them between 3.6 and 1.5 AU apart. The M-dwarf pair must
take at least 3700 years to make a circuit of the bright G star
(called "A"). Another faint 11th magnitude star lies some four
minutes of arc (at least 2100 AU) away. If a real companion, it is
an M dwarf that takes 67,000 years to orbit. There is also a
suggestion of a VERY close companion to Mu Her A (sometimes called
Aa), which seems to be a mere 2.2 AU distant. It may well not be
real. From Mu Her A the M dwarf pair would appear as much as half
a degree apart, each shining roughly as bright as our full Moon.
What a sight were suspected Aa not there and were there actually a
planet instead.